Ladder mechanism for an assembling apparatus for the manufacture of completely or partly finished packages of slats for venetian blinds

ABSTRACT

The application discloses a ladder mechanism for an assembling apparatus for the manufacture of at least partly finished packages of slats for venetian blinds of the type equipped with ladders comprising side cords and rungs therebetween and with means for lifting the ladders, said mechanism comprising movable means for directing rungs of a ladder to be placed alternately on one side and on the other side of the plane of the ladder, said movable means comprising two hinged rung positioning means such as a plate, brace and the like, each having a free end for cooperating with the rungs of a ladder, said free ends, when being in the inoperative position, being directed toward one another and overlapping one another as well as the plane of supply of the ladder in question, the rung positioning means being arranged and embodied such that during the operation of the assembling apparatus a rung always comes into contact with the free end of one of the positioning means and takes along said free end simultaneously with the overlapping free end of the second positioning means in its subsequent movement from the receiving position, said contact causing a lateral displacement of the rung, and that during the continued movement first the overlapping of the free end of the second positioning means is removed and said free end assumes its receiving position for cooperation with the next rung, and thereafter the firstmentioned rung is released from the free end of the respective positioning means which subsequently assumes the overlapping position. The apparatus also includes a side cord guiding and spreading means and a rung stretching means below the rung positioning means, said guiding and spreading means and the rung stretching means comprising two rung stretching and side cord spreading plates secured to a supporting plate and having edge guides diverging in the direction of the rung positioning means.

United States Patent 1191 Edixhoven 11 3,736,631 1451 June 5, 1973 [54] LADDER MECHANISM FOR AN ASSEMBLING APPARATUS FOR THE MANUFACTURE OF COMPLETELY OR PARTLY FINISHED PACKAGES OF SLATS FOR VENETIAN BLINDS Primary ExaminerAndrew R. Juhasz Assistant 'Examiner- Leon Gilden Attorney- Pennie, Edmonds, Morton, Taylor and Adams 571 ABSTRACT I The application discloses a ladder mechanism for an assembling apparatus for the manufacture of at least partly finished packages of slats lor venetian blinds of the type equipped with ladders comprising side cords and rungs therebetweenand with means for lifting the ladders, said mechanism comprising'movable means for directing rungs of a ladder to be placed alternately on one side and on the other side of the plane of the ladder, said movable means comprising two hinged rung positioning means such as a plate, brace and the like, each having a free end for cooperating with the rungs of a ladder, said free ends, when being in the inoperative position, being directed toward one another and overlapping one another as well as the plane of supply of the ladder in question, the rung positioning means being arranged and embodied such that during the operation of the assembling apparatus a rung always comes into contact with the free end of one of the positioning means and takes along said free end simultaneously with the overlapping free'end of the second positioning means in its subsequent movement from the receiving position, saidcontact causing a lateral displacement of the rung, and that during the continued movement first the overlapping of the free end of the second positioning means is removed and said free end assumes its receiving position for cooperation with the next rung, and thereafter the first-mentioned rung is released from the free end of the respective positioning means which subsequently assumes the overlapping position. The apparatus also includes a side cord guiding and spreading means and a rung stretching means below the rung positioning means, said guiding and spreading means and the rung stretching means comprising two rung stretching and side cord spreading plates secured to a supporting plate and having edge guides diverging in the direction of the rung positioning means.

10 Claims, 5 Drawing Figures PATENTEDJUH 5 1975 A'I'TORNEKS LADDER MECHANISM FOR AN ASSEMBLING APPARATUS FOR THE MANUFACTURE OF COMPLETELY OR PARTLY FINISHED PACKAGES OF SLATS FOR VENETIAN BLINDS means causing the successive rungs of a ladder to arrive alternately on one side and on the other side of a lifting means threaded or to be threaded through the successive passages situated one above the other of the slats.

The apparatus of the present invention is primarily regarded as an improvement on the machine or apparatus disclosed in Hunter Douglas, Peeters, U. S. Pat. No.

3,531,838, granted Oct. 6, I970. The apparatus of that patent includes certain mechanisms which may be made use of in connection with the present invention particularly with respect to means for lifting the ladder after its side cords have been moved apart and its rungs stretchedi The patent also discloses the production of stacks of slats for venetian blinds, in which such stacks are collected between a pair of guides after being individually placed between the side cords of a ladder or ladders and then lifted to the position of the stack. At the same time the individual slat is lifted in such a manner that it simultaneously lifts the ladder.

In this respect it is to be noted that, although the invention relates to a ladder mechanism for all ladders of the type of which in the stretched and spread condition of the ladder the rungsare situated in a flat plane, for a good understanding and a better readability in the specification and claims only the most current species of ladder of this type will be mentioned, namely the socalled ladder cord. Hence in the following text the terms ladder cord, side cords and rungs will be used.

' The ladder mechanisms known so far for or in apparatuses for the manufacture of completely or partly finished packages of slats for Venetian blinds, in which the rungs are situated alternately on one side and on the other side of the lift 'means present the drawback of being complicated and therefore very expensive. This applies for apparatuses of the type in which needles destined for the lift means, such as lift cords, are used, which are situated above the passages in the slats destined for the lift cords after introduction of the slats into the apparatus, as well as for apparatuses of the type in which the rungs are moved alternately to the right and to the left and said rungs are maintained in the position given to them by clamping them between the successive slats introduced into the apparatus.

The invention aims at providing a simple ladder mechanism which is suitable for many types of assembling apparatuses, such as among other things of the afore-mentioned types, and which notwithstanding its simplicity of design functions reliably.

This is accomplished with the ladder mechanism according to the invention in that the movable means of the ladder mechanism consist of two hinged rung positioning plates each having a free end for co-operation withrungs of a ladder, said free ends in the inoperative position being directed in opposite direction and each having a predetermined receiving position, the free end which is in the receiving position being overlapped by the free end of the other positioning means, in which position both free ends each overlap the plane of supply of the ladder in question, the positioning means being also arranged and embodied such that during the operation of the assembling apparatus a rung always comes into contact with the free end of one of the positioning means and takes along said free end simultaneously with the overlapping free end of the second positioning means in its subsequent movement from the receiving position, said contact causing a lateral displacement of the rung, and that during the continued movement first the overlapping of the free end of the second positioning means is removed and said free end assumes its receiving position for co-operation with the next rung, and thereafter the 'firstmentioned rung is rebased' from the free end of the respective positioning means which subsequently assumes the overlapping position. Because one positioning means overlaps the other positioning means in the receiving position and during part of the displacement of said other positioning means, the first-mentioned positioning means, with the other positioning means being :in the receiving position, can never come into contact with the rung situated below the other positioning means, and, because said first positioning'means is already released by the other positioning means before it releases the rung in question, said one positioning means cannot come into contact either with said rung during the movement of the other positioning means. This applies also for the other positioning means when being in the overlapping position after the first-mentioned positioning means is brought into the receiving position. The operation of the positioning means thereby takes place in the usual ways by moving upwards the entire ladder or the respective part of the ladder each time when a slat introduced into the apparatus is moved upwards.

The positioning means may have any shape and may, for instance, consist of rods or strips. According to the invention they preferably consist each of a plate or brace the free end of which is bent or cranked. In this instance only two positioning means are necessary having a width which is smaller than the length of a rung. When a rung comes into contact with such a positioning means, said rung moves automatically towards the sharp bend of the positioning means. As a result thereof the rung is not only reliably .held, but it is moved, together with the ladder portion above said rung, in the same lateral direction so that at least the rung above it also moves in said direction and is kept in said direction When the rung is released by the respective positioning means the other positioning means has already assumed the receiving position for co-operation with the underlying rung and by this co-operation this following rung is moved in a lateral direction which is opposite to the lateral displacement of the preceding rung. In this way the higher rungs assumesuccessively the correct position, for example on either side of a needle or on either side of the successive slat passages.

According to the invention the positioning means may be moved into their receiving position by their own weight, by an extra weight or in some other known way. Preferably, however, the positioning means are spring-biased in a direction towards their receiving position. As a spring one may use a drawspring, a spring mounted on the pivot, etc.

It is still to be noted that in a number of existing assembling apparatuses hinged stretching plates are used which are also spring-biased and co-operate with the successive transverse connections or rungs of the ladder. However, all these stretching plates only have the function of keeping the ladder taut. None of these known constructions presents in any way a hint which may lead to the simple and efficient solution which the invention offers for the existing problem of the soc'alled weaving of the lift means through the rungs of the ladder.

A preferred embodiment of the ladder mechanism according to the invention is characterized in that the receiving position of the positioning means is established by a stop having two steps, one step of which cooperates with a portion remote from the free end of one positioning means and the other step with the free end of the other positioning means. Thus one is certain to accomplish that the positioning means always assume the receiving position while overlapping each other in the correct way.

According to the invention the ladder mechanism may comprise a guiding means for the ladder, which is embodied as a stretching and spreading mechanism for the ladder disposed in the main direction of movement of the ladder before the rung positioning means, said stretching and spreading mechanism comprising several stretching and spreading means for guiding and spreading the side cords of the ladder, said stretching and spreading means defining a space which widens in the direction of the rung positioning means and having at their broad end a width substantially equal to the length of the rungs, said stretching and spreading means constituting two uninterrupted guiding gaps which each of them are smaller than the thickness of a ladder side cord and serve to let the rungs pass and to guide them and to guide and spread the side cords.

The stretching and spreading means may thereby comprise two stretching and spreading plates secured to a supporting plate and each being provided with an edge bent towards the supporting plate, said two edges being situated at the sides of the stretching and spreading plates facing each other and extending obliquely or according to a curve such that they define a space diverging in the direction of the rung positioning means,

the guiding gaps being situated between the bent edges and the supporting plate. Said stretching and spreading plates may be influenced by a spring pressure urging the bent edges against the supporting plate. Thus the stretching and spreading plates may be adapted in a simple way to the thickness of the ladder. Each of the stretching and spreading plates may be secured to the supporting plate with the aid of a spacer pin and two bolts and nuts provided with a pressure spring. By arranging the spacer pins and the bolts in slots it is possible to adjust the stretching and spreading plates also in a simple manner to the desired ladder width.

The supporting plate of the stretching and spreading plates may, according to the invention, be stepped at its upper end and said stepped upper end may constitute mon base plate which is releasably secured to the assembling apparatus. Consequently this ladder mechanism can be manufactured as one integral part of the assembling apparatus.

The invention will be further explained below with reference to the accompanying drawings showing by way of example an embodiment of the ladder mechanism according to the invention.

The drawings show in:

FIG. 1 is a front view at right angles to the plane of the ladder of the ladder mechanism in the receiving position of a rung, and above said mechanism a slat receiving means and a needle schematically shown for simultaneously or subsequently providing a lift cord for the ladder;

FIG. 2 is a broken view similar to that of FIG. 1 showing the ladder mechanism in the position in which one positioning plate which has caught the rung has released the other positioning plate;

FIG. 3 is a broken view similar to that of FIG. 2 showing the ladder mechanism in the position in which the rung has been releasedby one positioning plate and the other positioning plate is in the receiving position;

FIG. 4 is a front view of said ladder and its spreading mechanism; and

FIG. 5 is a section according to the line V-V of FIG. 4.

The ladder mechanism 1 shown in the drawings comprises a base plate 2 in which two pivot pins 3,4 are secured. On each of said pivot pins there is provided a positioning plate 5,6 with its free end in the path of the rungs 7 of a ladder with side cords 8,9, said positioning plates being caught in turn by a rung 7 and, after having been turned according to a certain angle, releasing said rung 7. The free end of each positioning plate comprises an end portion 10,11 bent or cranked according to an angle. The positioning plates 5,6 have such a length that in their receiving position they project beyond the vertical center plane between the pivot pins 3,4 and they are adapted to pass each other before a positioning means, which is caught by a rung and swung upwards, releases said rung. Consequently, when one positioning means is in thereceiving position, the end portion of other positioning means will be situated on the end portion of the said one positioning means. In the receiving position the positioning means 6 is situated on a stop 12 located at a distance from its free end, whereas the positioning means 5 in its receiving position abuts with its free end against a lower stop 13. The positioning plates are put into their receiving position by draw-springs 23 and 24 fastened to the base plate.

In the embodiment of the ladder mechanism 1 as shown in the drawings the stops 12 and 13 are constituted by the upper portion of a supporting plate 14, said upper portion being stepped.

On said supporting plate 14 (FIGS. 4 and 5) there are provided two stretching and spreading plates 15,16 for the ladder, by means of which the rungs 7 of the ladder are stretched so that a slat may be received in a flat condition between the side cords of the ladder. For this purpose each stretching and spreading plate comprises a rectangularly bent end portion 17,18 the ends of which are situated at such a distance from the supporting plate 14 as to form gaps through which the rungs may pass whilst the side cords 8,9 of the ladder are retained.

Each stretching and spreading plate is secured to the supporting plate 14 with the aid of a spacer pin 19 and two bolts 21 with nuts provided with a compression spring'20. With the aid of the spacer pins 19 and the bolts 21 one may adjust the width of the gaps between the supporting plate 14 and the ends of the portions 17 and 18.

As is apparent from FIG. 4 each stretching and spreading plate has a sloping edge portion such that the distance between said edge portions at the lower end where the ladder enters the stretching and spreading plates increases towards the upper end. As a result thereof the rungs 7 of the ladder are spread upon leaving the stretching and spreading plates and .will be caught by a positioning plate 5 and 6respectively in the receiving position.

it is obvious that the invention is not restricted to the ladder mechanism as described above by way of example and shown in the drawings and that this ladder mechanism can be applied in any type of an apparatus for the manufacture of entirely or partly finished packages of slats for Venetian blinds comprising ladders of the type of which in the stretched and spread position of the ladder the rungs are situated in one flat plane, if

the rungs of the ladders should be positioned alternately on one or on the other side of the respective lifting means. Instead of positioning means for the rungs consisting of a plate also brace-like positioning means may be used, and it is also possible to use bipartite or multipartite positioning means instead of positioning means consisting of one single plate or brace.

In FIG. 1, the portion of the apparatus above the members 5 and 6, for positioning the rungs of the ladder, comprises spaced slat guides 25 secured to the upper part of the base plate 2. The guides 25 as seen are part of a structure in which similar upright guide members are located directly in back of the guide members 25 so as to retain and hold a group of slats 26 along with a cord ladder, the rungs 7 of which alternate with the slats of the group 26. The arrangement is similar to that shown and described in the above U. S. patent, and each of the guide members 25 is provided with inwardly-projecting spring-biased catches or stops 27'on which the group of slats is supported.

In the operation of the apparatus, the slats, such as the slat 28, are inserted one at a time between the opposed upright slat guides and also between adjacent rungs 7. The slat 28 rests on pins 29 projecting forward from lifting blocks 30. Each block is provided with a lifting means 31; and, when the slat is in place and the means 31 are lifted, the slat is elevated along with the ladder to the bottom position of the group of slats 26 so that it rests on the catches or stops 27.

Each of the slats is provided with an aperture 32 so that if desired a needle 33 may be passed through all of the apertures and connected by a lifting cord to the ladder or the slat which rests on the lifting pins 29. If a lifting cord is employed, it will not be necessary to use the lifting means 31.

I claim:

l. A ladder mechanism for an assembling apparatus forthe manufacture of at least partly finished packages of slats for Venetian blinds of the type equipped with ladders comprising side cards and rungs therebetween and with means for lifting the ladders, said mechanism comprising movable means causing the successive rungs of a ladder to arrive alternately on one side and on the other side of the plane of the ladder, said movable means comprising two hinged rung positioning means such as a plate, brace and the like, each having a free end for co-operating with the rungs of a ladder, said free ends, when being in the inoperative position, being directed towards one another and overlapping one another as well as the plane of supply of the ladder in question, the rung positioning means being arranged and embodied such that during the operation of the assembling apparatus a rung always comes into contact with the free end of one of the positioning means and takes along said free end simultaneously with the overlapping free end of the second positioning means in its subsequent movement from the receiving position, said contact causing a lateral displacement of the rung, and that during the continued movement first the overlapping of the free end of the second positioning means is removed and said free end assumes its receiving position for co-operation with the next rung, and thereafter the first-mentioned rung is released from the free end of the respective positioning means which subsequently assumes the overlapping position. i

2. A ladder mechanism according to claim 1, characterized in that the rung positioning means are springbiased in a direction towards their receiving position, the free end of each of said rung positioning means being bent towards said receiving position, the receiving position of each of said rung positioning means being defined bya stop.

3. A ladder mechanism according to claim 1, characterized in that the rung positioning means are springbiased in a direction towards their receiving position, the free end of each of said rung positioning means being bent towards said receiving position, the receiving position of each of said rung positioning means being defined by a stop, having two steps, one of said steps co-operating with a portion remote from the end of one rung positioning means and the other of said steps co-operating with the end of the other rung positioning means.

4. A ladder mechanism for an assembling apparatus for the manufacture of at least partly finished packages of slats for Venetian blinds of the type equiped with ladders comprising side cardsand rungs therebetween and with means for lifting the ladders, said mechanism comprising movable means causing the successive rungs of a ladder to arrive alternately on one side and on the other side of the plane of the ladder, said movable means comprising two hinged rung positioning means such as a plate, brace and the. like, each having a free end for co-operating with the rungs of a ladder, said free ends, when being in the inoperative position, being directed towards one another and overlapping one another as well as the plane of supply of the ladder in question, the rung positioning means being arranged and embodied such thatduring the operation of the assembling apparatus a rung always comes into contact with the free end of one of the positioning means and takes along said free end simultaneously with the overlapping free end of the second positioning means in its subsequent movement from the receiving position, said contact causing a lateral displacement of the rung, and that during the continued movement first the overlap ping of the free end of the second positioning means is removed and said free end assumes its receiving position for co-operation with the next rung, and thereafter the first-mentioned rung is released from the free end of the respective positioning means which subsequently assumes the overlapping position, said ladder mechanism further comprising a guiding means defining the plane of the ladder supply, said guiding means being provided with a stretching and spreading mechanism for the ladder disposed in the main direction of movement of the ladder before the rung positioning means, said stretching and spreading mechanism comprising stretching and spreading means for guiding and spreading the side cords of the ladder, said stretching and spreading means defining a space which widens in the direction of the rung positioning means and having at their broadend a width substantially equal to the length of the rungs, said stretching and spreading means constituting means providing two uninterrupted guiding gaps each of them being smaller than the thickness of a ladder side cord and serve to let the rungs pass and to guide them and to guide and spread the side cords. 5. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means comprise two stretching and spreading plates secured to a supporting plate and each being provided with an edge 1 bent towards the supporting plate, said two edges being situated at the sides of the stretching and spreading plates facing each other and defining a space diverging in the direction of the rung positioning means, the guiding gaps being situated between the bent edges and the supporting plate.

6. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means comprise two stretching and spreading plates secured to a supporting plate and each being provided with an edge bent towards the supporting plate, said two edges being situated at the sides of the stretching and spreading plates facing each other and defining a space diverging in the direction of the rung positioning means, the guiding gaps being situated between the bent edges and the 8 supporting plate, the stretching and spreading plates being provided with spring means urging the bent edges toward the supporting plate.

7. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means comprise two stretching and spreading plates secured to a supporting plate and each being provided with an edge bent towards the supporting plate, said two edges being situated at the sides of the stretching and spreading plates facing each other and defining a space diverging in the direction of the rung positioning means, the guiding gaps being situated between the bent edges and the supporting plate, the stretching and spreading plates being provided with spring means urging the bent edges toward the supporting plate, each of the stretching and spreading plates is secured to the supporting plate with the aid of a spacer pin and two bolts and nuts provided with a pressure spring.

8. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means is secured to a supporting plate'which is stepped at its upper end, said stepped upper end constituting the stops for the rung positioning means.

9. A ladder mechanism according to claim 4, characterized in that each of the stretching and spreading means comprise two spring loaded stretching and spreading plates defining the diverging ladder guiding gaps, each of said stretching and spreading plates being secured to the supporting plate by means of a spacer pin and two bolts.

10. A ladder mechanism according to claim 4, characterized in that the rung positioning means, the springs for said rung positioning means and the ladder guiding means are mounted on a common base plate which is releasibly secured to the assembling apparatus. i 

1. A ladder mechanism for an assembling apparatus for the manufacture of at least partly finished packages of slats for Venetian blinds of the type equipped with ladders comprising side cards and rungs therebetween and with means for lifting the ladders, said mechanism comprising movable means causing the successive rungs of a ladder to arrive alternately on one side and on the other side of the plane of the ladder, said movable means comprising two hinged rung positioning means such as a plate, brace and the like, each having a free end for cooperating with the rungs of a ladder, said free ends, when being in the Inoperative position, being directed towards one another and overlapping one another as well as the plane of supply of the ladder in question, the rung positioning means being arranged and embodied such that during the operation of the assembling apparatus a rung always comes into contact with the free end of one of the positioning means and takes along said free end simultaneously with the overlapping free end of the second positioning means in its subsequent movement from the receiving position, said contact causing a lateral displacement of the rung, and that during the continued movement first the overlapping of the free end of the second positioning means is removed and said free end assumes its receiving position for cooperation with the next rung, and thereafter the first-mentioned rung is released from the free end of the respective positioning means which subsequently assumes the overlapping position.
 2. A ladder mechanism according to claim 1, characterized in that the rung positioning means are spring-biased in a direction towards their receiving position, the free end of each of said rung positioning means being bent towards said receiving position, the receiving position of each of said rung positioning means being defined by a stop.
 3. A ladder mechanism according to claim 1, characterized in that the rung positioning means are spring-biased in a direction towards their receiving position, the free end of each of said rung positioning means being bent towards said receiving position, the receiving position of each of said rung positioning means being defined by a stop, having two steps, one of said steps co-operating with a portion remote from the end of one rung positioning means and the other of said steps co-operating with the end of the other rung positioning means.
 4. A ladder mechanism for an assembling apparatus for the manufacture of at least partly finished packages of slats for Venetian blinds of the type equiped with ladders comprising side cards and rungs therebetween and with means for lifting the ladders, said mechanism comprising movable means causing the successive rungs of a ladder to arrive alternately on one side and on the other side of the plane of the ladder, said movable means comprising two hinged rung positioning means such as a plate, brace and the like, each having a free end for co-operating with the rungs of a ladder, said free ends, when being in the inoperative position, being directed towards one another and overlapping one another as well as the plane of supply of the ladder in question, the rung positioning means being arranged and embodied such that during the operation of the assembling apparatus a rung always comes into contact with the free end of one of the positioning means and takes along said free end simultaneously with the overlapping free end of the second positioning means in its subsequent movement from the receiving position, said contact causing a lateral displacement of the rung, and that during the continued movement first the overlapping of the free end of the second positioning means is removed and said free end assumes its receiving position for co-operation with the next rung, and thereafter the first-mentioned rung is released from the free end of the respective positioning means which subsequently assumes the overlapping position, said ladder mechanism further comprising a guiding means defining the plane of the ladder supply, said guiding means being provided with a stretching and spreading mechanism for the ladder disposed in the main direction of movement of the ladder before the rung positioning means, said stretching and spreading mechanism comprising stretching and spreading means for guiding and spreading the side cords of the ladder, said stretching and spreading means defining a space which widens in the direction of the rung positioning means and having at their broad end a width substantially equal to the length of the rungs, said stretching and spreading means constituting means providing two uninterrupted guiding gaps each of them being smaller than the thickness of a ladder side cord and serve to let the rungs pass and to guide them and to guide and spread the side cords.
 5. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means comprise two stretching and spreading plates secured to a supporting plate and each being provided with an edge bent towards the supporting plate, said two edges being situated at the sides of the stretching and spreading plates facing each other and defining a space diverging in the direction of the rung positioning means, the guiding gaps being situated between the bent edges and the supporting plate.
 6. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means comprise two stretching and spreading plates secured to a supporting plate and each being provided with an edge bent towards the supporting plate, said two edges being situated at the sides of the stretching and spreading plates facing each other and defining a space diverging in the direction of the rung positioning means, the guiding gaps being situated between the bent edges and the supporting plate, the stretching and spreading plates being provided with spring means urging the bent edges toward the supporting plate.
 7. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means comprise two stretching and spreading plates secured to a supporting plate and each being provided with an edge bent towards the supporting plate, said two edges being situated at the sides of the stretching and spreading plates facing each other and defining a space diverging in the direction of the rung positioning means, the guiding gaps being situated between the bent edges and the supporting plate, the stretching and spreading plates being provided with spring means urging the bent edges toward the supporting plate, each of the stretching and spreading plates is secured to the supporting plate with the aid of a spacer pin and two bolts and nuts provided with a pressure spring.
 8. A ladder mechanism according to claim 4, characterized in that the stretching and spreading means is secured to a supporting plate which is stepped at its upper end, said stepped upper end constituting the stops for the rung positioning means.
 9. A ladder mechanism according to claim 4, characterized in that each of the stretching and spreading means comprise two spring loaded stretching and spreading plates defining the diverging ladder guiding gaps, each of said stretching and spreading plates being secured to the supporting plate by means of a spacer pin and two bolts.
 10. A ladder mechanism according to claim 4, characterized in that the rung positioning means, the springs for said rung positioning means and the ladder guiding means are mounted on a common base plate which is releasibly secured to the assembling apparatus. 